Offsite construction key to major UK Passivhaus refit

A £2.8 million residential refurbishment to the EnerPHit standard is part of the European E2Rebuild initiative. Mark Elton reports.

The use of prefabricated elements is helping to minimise disruption to residents and control costs on one of the UK’s largest Passivhaus retrofits. The Parkview Hub development, on London’s Thamesmead estate, is being refurbished by Gallions Housing Association to the Passivhaus Institute-certified EnerPHit standard.

The five-storey scheme currently has nine 2/3-bed maisonettes and nine 4-bed maisonettes stacked above ground level garages, but retrofit proposals will see the garages converted to retail and community uses, as well as adding an additional room to lower maisonettes. Dutch Passivhaus consultant Trecodome was responsible for integrating the scheme into the Europe-wide E2Rebuild initiative, which promotes the use of offsite construction methods in deep retrofits as a way of reducing cost and disruption to residents.

Timber cassette approachDating from the early 1970s and constructed from precast concrete sandwich panels, the thermal performance of the existing structure is poor, meaning residents suffer from high fuel costs and condensation. The retrofit is being implemented using an innovative prefabricated timber cassette approach developed by contractor Gumpp & Maier UK.

Based on precise survey information, external wall components are assembled in the contractor’s Bavarian factory from 280mm deep timber stud cassettes filled with cellulose insulation. They are fitted with triple-glazed windows and doors from Rationel and random-width timber cladding boards. The overall cladding strategy and all junction details were designed in close collaboration with architect sustainableBYdesign, which also undertook thermal bridge analysis to fine tune the thermal performance of panels and interfaces.

The finished timber cassettes, which typically span two properties wide by one storey high, are transported to site and craned onto pre-positioned load-bearing battens, taking a matter of days to complete a facade. Existing PVC windows and spandrels are then removed internally and the zone between new cladding and concrete facade insulated and lined to give a worst case U-value of less than 0.15W/m²K.

The final intervention is the retrofit of the Zehnder heat recovery ventilation system and replacement boilers, to designs by engineering consultant Alan Clarke, all carried out with residents in situ.

At ground level, former parking bays will have overhead and floor insulation installed and new Rheinzink shingle clad facades to provide super-insulated space suitable for subsequent fit-out as retail, community and workspace facilities – all part of the wider regeneration of the Thamesmead area. Roof-mounted photovoltaics will feed low-carbon electricity to these areas, delivering attractive low carbon retail and community facilities to the market.

The retrofit construction cost of £2.8 million reflects not only the exemplary standard which the scheme is targeting but the extensive additional space that it creates in the heart of a neighbourhood previously lacking such resources.

Sign up for a previewThe Parkview Hub enerPHit scheme is an exemplar of energy efficient refurbishment, forming the exception rather than the rule. Another EU funded project – EuroPHit – aims to make projects like this the norm by establishing cost effective and step-by-step routes to achieve similar standards of refurbishment, thus making it more widely attainable.

The project is due to complete early next year but the industry can get a preview of the scheme when it opens to visitors as part of the Europe-wide 2013 Passive House Open Days. Parkview Hub will be open on Friday 8 November at 11:00 and 14:00, but places are limited and require pre-booking. Please email tilforda@bre.co.uk.to book your place. Visits are expected to last around one hour.